Simulated Recruitment of Medial Rectus Motoneurons by Abducens
InternuclearNeurons: Synaptic Specificity versus Intrinsic Motoneuron
Properties
Paul Dean
Paul Dean, Department of Psychology, University of Sheffield, Sheffield
S102TP, England, FAX: +44 (0) 114 276 6515, PHONE: +44 (0) 114 222 6521, e-
mail: P.Dean@sheffield.ac.uk
APStracts 4:0080N, 1997.
ABSTRACT
Ocular motoneuron firing rate is linearly related to conjugate eye
position with slope K, above recruitment threshold q. Within the population of
ocular motoneurons K increases as q increases. These differences in firing-
rate between motoneurons might be determined either by the intrinsic
properties of the motoneurons, or by differences in synaptic input to them, or
by a combination of the two.This question was investigated by simulating the
input signal to medial rectusmotoneurons (MR-MNs) from internuclear neurons of
the abducens nucleus (INNs).INNs were represented as input nodes in a two
layer neural-net, each with weighted connections to every output node
representing an MR-MN. Individual simulated MR-MNs were assigned parameters
corresponding to an intrinsic current threshold and an intrinsic frequency-
current (f-I) slope . Their firing rates were calculated from these
parameters, together with the effective synaptic current produced by their
synaptically weighted INN inputs,using assumptions employed in computer
simulations of spinal motoneuron pools.The experimentally observed firing
rates of MR-MNs served as training data for the net. Two training conditions
were used. (i) Synaptic weights were fixed, and the intrinsic parameters of
the MR-MNs allowed to vary, corresponding to the situation where each MR-MN
receives a common synaptic drive. (ii) Intrinsic MR-MN properties were fixed,
and synaptic weights were allowed to vary. In each case, the varying
quantities were trained with a form of gradient descent error-reduction.The
simulations revealed three problems with common-drive model. (i) The
recruitment of INNs produced nonlinear responses in MR-MNs with low qs. (ii)
The range of intrinsic current thresholds required to reproduce the
observedrange of q were generally larger than those measured experimentally
for cat ocular motoneurons. (iii) The intrinsic f-I slope g increased with .
Experimental data from cat indicates that g decreases with . When synaptic
weights were allowed to vary, all three problems with the common-drive model
were overcome. This required MR-MNs receiving selective input from INNs with
similar firing-rate thresholds.These results suggest that the differences in
firing-rate properties among MR-MNs in relation to steady-state eye position
cannot be derived from their intrinsic properties alone, but result at least
partly from differences in their synaptic inputs. An MR-MN's individual set
of synaptic inputs constitutein effect a premotor receptive field.
Received 1996 Decemeber 16; accepted in final form 1997 May 14.
APS Manuscript Number J982-6
Article publication pending J. Neurophysiol.ISSN 1080-4757
Published in APStracts on 11 June 1997
Copyright 1997 The American Physiological Society.